Electronic transducer for measuring flexion

ABSTRACT

An electronic flexion transducer ( 10 ) comprises a helical coil formed from two interposed electrical conductors (A) and (B) which are electrically isolated from each other either by an insulated coating on the conductors or by being embedded in a dielectric material ( 11 ) having a substantial elasticity. The coil is bonded along one side over its entire length to a flexible non-extensible support surface ( 13 ) whereby flexion of the support surface causes a change in the length of the coil between said one side, which retains its original length, and a side opposite to said one side, which either increases or decreases in length. This relative change in length between opposite sides causes angular separation between adjacent turns of the coil and since adjacent turns represent respective conductors (A) and (B) there is a change in capacitance between the respective conductors which can be used to provide an electrical signal to a computer to record and display the change. A puppeteer&#39;s glove incorporating said transducer in each finger is also described.

CROSS REFERENCE TO RELATED APPLICATION

The present application is the U.S. national stage application ofInternational Application PCT/AU00/00383, filed Apr. 27, 2000, whichinternational application was published on Nov. 9, 2000 as InternationalPublication WO 00/66970 in the English language. The InternationalApplication claims priority of Australian Patent Application PQ 0047,filed Apr. 29, 1999.

SUMMARY OF THE INVENTION

The present invention relates to an electronic transducer for measuringflexion of bodies or structures and particularly bodies or structureswhich are soft or elastic and which may be subject to length variationsduring flexion. The invention has particular utility in the measurementof flexion of joints in the human body without introducing constraintsbut it will be apparent that the invention has wide application and isin no way limited to this particular purpose.

In one practical embodiment of the invention transducers according tothe invention are mounted in or on the fingers of a glove whereby humanfinger motion is converted to data streams for input to a computer. Sucha glove is used in professional animation production by Puppeteers.

One prior art transducer device in the form of a Puppeteer's glove usesfibre-optic cables that circumscribe each finger of the glove throughpre-sewn channels and transmit light from Light Emitting Diodes (LED's)at one end of the cable to photo receptors at the other end of thecable. The variation in bending of the fibre-optic cable causes avariation in the light received by the receptor and thus flexion of thefingers is able to create data streams related thereto. A number ofproblems are inherent in these prior art devices. For example, thefibre-optic cables required for the purpose are communication gradecables which are expensive and, although flexible, are not designed forcontinuous flexion and consequently they have a short life span. Otherproblems such as connecting power to the glove and the electroniccircuit components required for implementation render the glovescumbersome and expensive to repair.

A further prior art device is described in U.S. Pat. No. 5,090,248entitled “Electronic Transducer” and comprises an extensometer in theform of two or more interposed helical coils configured to allow elasticdeformation thereof and preferably mounted in an elastic dielectricmaterial which provides a restoring force to return the conductors totheir original configuration after deformation. The deformation causes achange in the capacitance between the conductors (by changing theinter-electrode gap between the respective coils) and this change ismonitored to measure dimensional change over a wide dynamic range. Thedimensional change is essentially elongation or when the extensometer isattached to a movable joint is a combination of elongation anddeflection which cannot be separated since the change in capacitance isa measure of the changes in inter-electrode's gap due to both elongationand deflection. Therefore this prior art transducer is not suitable formeasuring angular deflections reliably or accurately as would berequired, for example, in the aforementioned Puppeteer's glove or in anyother situations where accurate and reproducible data is required.

BRIEF DESCRIPTION OF THE INVENTION

Accordingly, it is an object of this invention to provide an electronicflexion transducer that overcomes one or more of the aforementionedproblems, or other problems, associated with known transducers when usedfor measuring or monitoring flexion.

The invention provides an electronic flexion transducer comprising twoor more interposed electrical conductors forming an elongate coil, saidcoil being bonded on one side along its length to a flexiblenon-extensible support surface whereby flexion of the surface causes achange in length of the coil between said one side, which retains itsoriginal length and a side opposite to said one side, thereby causing anangular separation between said conductors with a consequent change incapacitance of the coil.

BRIEF DESCRIPTION OF THE DRAWING

In order that the invention may be more readily understood a particularembodiment will now be described with reference to the accompanyingdrawings wherein:

FIG. 1(a) is a sectional side view of a flexion transducer according tothe invention;

FIG. 1(b) is a perspective view from above of a flexion transduceraccording to the invention and as depicted in FIG. 1(a);

FIG. 1(c) is an enlarged view of an end portion of the transducer ofFIG. 1(b);

FIGS. 1(d), (e), (f) and (g) show several alternative cross sections ofthe transducer core windings of the transducer shown in FIG. 1(a);

FIGS. 2(a) and (b) are similar to FIG. 1(a) but shows the transducer ina relaxed and flexed state respectively;

FIGS. 2(c) and (d) represent a pair of adjacent turns of the coils ofthe transducer in the relaxed and flexed state shown as their equivalentcapacitances symbolised as parallel plate capacitors with differentgaps;

FIG. 3 is a schematic view of the back of a glove showing transducersaccording to the invention mounted on each finger and the thumb of theglove;

FIG. 4 is a schematic block diagram of a general oscillator circuit forproducing a digital and analog data output from a varying capacitance.

FIG. 5 is a circuit diagram similar to FIG. 4 when applied to thetransducer of this embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The flexion transducer 10 is shown in the drawings to comprise a pair ofcoaxial interposed helical conductors A and B embedded in an elasticdielectric material 11 and to this point may be identical to thetransducer disclosed in U.S. Pat. No. 5,090,248. The transducer may beformed by winding the two conductors A and B as a bifilar coil over amandrel (not shown) to form an elongate helical coil. The mandrel isthen withdrawn and the coil is either embedded in dielectric material 11having a substantial elasticity or coated with said dielectric material.In either case the respective conductors are electrically isolated fromeach other.

After formation of the embedded or coated conductors A and B has beencompleted, the transducer is bonded on one side along its length to aflexible non-extensible base surface 13 which extends along the entirelength of the transducer. The bonding is achieved by means of a flexibleadhesive 12 such as silicon elastomer. The ends 17 of the respectiveconductors A, B extend out from the dielectric material to a terminationblock 18 FIG. 1(c) for interconnection to transducer read-out circuitryto be described hereinbelow. As is shown in FIG. 1(a), there is anequivalent capacitance CL between the coils of the conductors A and B.

Referring to FIG. 2(b) the transducer is shown in a flexed state inwhich case the side of the embedded coil that is attached to the supportsurface 13 remains at the same length as the undistorted transducer(represented by Lo in this Figure) whereas the opposite side of the coilstretches due to the larger radius of curvature. The net result is thatthe respective turns or loops of the conductors A and B separate on theouter side of the flexed transducer.

The sum of the angular separations between adjacent turns is equal tothe flexion of the transducer defined as the angle formed by thetangents to the transducer arc at the two ends.

FIGS. 2(c) and (d) show adjacent loops 26 & 27 respectively of therespective conductors A and B in the relaxed or non-flexed state and inthe flexed state with the respective equivalent capacitances 25symbolised as parallel plate capacitor with different gaps.

The capacitance between the respective conductors A and B changes whenthe transducer is flexed but the length of the transducer, at leastalong the edge attached to the support surface 13 does not change.

The change in capacitance between the respective conductors A and B is aconvenient measure of flexion and can easily be calibrated with the helpof a goniometer attached to the two end of the transducer.

FIGS. 1(e), (f) and (g) show different cross sections which can beobtained by deformation of the circular cross-section shown in FIG. 1(d)or by directly forming over a mandrel of appropriate cross-section. Theadvantage of using cross-section as shown in FIGS. 1(e) and (g) over thecircular cross-section of the same perimeter is that more of therespective electrodes A and B are at a larger radius of curvature andtherefore result in a larger change of capacitance for the same flexion,thereby increasing the transducer inherent sensitivity.

The transducer of FIGS. 1 and 2 can be attached to a stretch fabric (notshown) by means of adhesive or by stitching which is applied at each endof the transducer or alternatively along the entire length thereof. Thefabric may be stretch fabric forming part of a glove 16 as shown in FIG.3. In such a case, the transducer is attached along a finger 19 or thumb22 of the glove, so as to be on the back of the user's hand when theglove is worn. In an alternative embodiment the transducers are attachedon the palm side of the glove. A separate transducer 10 is attachedalong each finger and the thumb of the glove 16 although more than onetransducer may be attached on each. In a further alternative embodimentof a puppeteer's glove (not shown) a separate transducer may extend overeach knuckle joint of each finger or only over one or more of theknuckle joints. Electrical wires 20 extend from each transducer read-outcircuitry to a termination block 18 so that external circuitry may beconnected to the respective conductors of each transducer.

As will be evident, the relative change in length between the two sidesof the transducer causes an angular separation between the coils orturns of the conductors A and B. This angular separation produces achange in the capacitance of the transducer which is directly related tothe flexion of the support surface 13. With appropriate calibration, theangle of flexion, or joint rotation, can be accurately derived from themeasured change in capacitance. One embodiment of a transducer read-outcircuit will now be described with reference to FIGS. 4 and 5. Thetransducer capacitance C(θ) is arranged to be the timing capacitance ofan oscillator 23 such as a C-Mos 555 in the astable configuration asshown in FIG. 4. Flexing of the transducer modulates the outputfrequency of the oscillator.

For a parallel plate capacitor the capacitance is expressed by therelation

C=θS/(d+do)

where S is the effective area of the electrodes, d is the variabledistance between them and do is the fixed minimum separation due, forexample, to the thickness of insulation of the electrodes wires.

The same expression applies to the capacitance of the transducer in theflexed state where S represents the effective area of the electrodes Aand B and d is the equivalent separation which is to good approximationa linear function of the flexion angle θ, (d=kθ) as can be easilyinferred from FIGS. 2(b)-(2(d).

The capacitance of the transducer is therefore to good approximation

C(θ)=θS/(kθ+do).

The output frequency of the oscillator 23 is given by

Fout=a/RC

Where a is a circuit constant and R the feedback resistance.

Substituting in the above relation the expression of the transducercapacitance as function of flexion results in an essentially lineardependence of the output frequency as function of flexion:

Fout=Kθ+Fo

The parallel plate capacitor with variable gap is most sensitive tochanges in separation at small gaps.

The effective gap of the transducer even for very large flexion isalways very small, thereby assuring operation in the most sensitiveregion of its transfer characteristic, unlike the aforementionedtransducer disclosed in U.S. Pat. No. 5,090,248.

The output frequency Fout can be converted into an analogue current orvoltage using simple circuitry (FIG. 5). A micro-controller 24 is usedto perform all functions required to convert the changes in capacitance,into a form easily displayed and readable by an external instrument orcomputer. The oscillator or micro-controller 24 is incorporated in thetermination block 18 directly at one end of the transducer coils tominimise stray capacitances and to facilitate connection to externaldevices (see FIG. 1(c)). Examples of the oscillator or micro-controllercircuit are shown in FIG. 5.

A transducer such as that disclosed in aforementioned U.S. Pat. No.5,090,248 would not work in a situation measuring flexion angle or jointrotation since it would be subject to both elongation and deflectionwhich, in analysing the result, cannot be separated. The change incapacitance with this prior art transducer is a measure of the changesof inter-electrodes gap due to both elongation and angular deflection.With the transducer of the present invention, angular deflections onlyare transferred to the electronic transducer, via the flexiblenon-extensible support. The support surface 13 is attached to the bodyor joint in a way that facilitates possible length changes in the bodyitself. In other words, the elastic material of glove 16 may change inlength over the finger portion but the support surface 13 prevents alike change in length of the transducer 10. Since the flexion istransmitted to the transducers via the flexible non-extensible supports,exact positioning is not required and the capacitance changes areindependent of the size of the hand inside the glove and on shifts ofthe glove during flexing activity. This results in a reliable andreproducible data entry device for the remote control of computerprograms for general devices and instruments. In the case of apuppeteer's glove, it enables accurate monitoring of the flexions of thejoints and fingers of the hand and thus provides reliable data fortransmission to a remote computer for controlling animations.

Whilst the flexion transducer has been described in a practicalembodiment in relation to a glove, it will be evident that it has manyother applications and is therefore not limited to this particularpurpose. The transducer is suitable for use in any location whereflexion is to be measured in an accurate manner.

What is claimed is:
 1. An electronic flexion transducer comprising twoor more electrically isolated and interposed electrical conductorsforming an elongate coil, characterized in that said coil is bonded onone side along its length to a flexible non-extensible support surfacewhereby flexion of the surface causes a change in length of the coilbetween said one side, which retains its original length and a sideopposite to said one side, thereby causing an angular separation betweensaid conductors with a consequent change in capacitance of the coil. 2.A transducer as defined in claim 1, characterized in that, said coil isenclosed in an elastic dielectric material forming a casing for saidcoil, said casing being bonded to said support by means of a flexibleadhesive.
 3. A transducer as defined in claim 2, characterized in that,said coil comprises two said conductors, and appropriate insulated wiresare connected to the respective conductors to facilitate monitoring ofthe change in capacitance due to said flexion whereby, with appropriatecalibration, the angle of flexion, or joint rotation, is accuratelyderived.
 4. A transducer as defined in claim 3, characterized in that,the cross-sectional shape of said coils is circular.
 5. A transducer asdefined in claim 3, characterized in that, the cross-sectional shape ofsaid coil is rectangular, with unequal sides.
 6. A transducer as definedin claim 5, characterized in that said one side coincides with a shortside of said rectangular coil.
 7. A transducer as defined in claim 5,characterized in that said one side coincides with a long side of saidrectangular coil.
 8. A transducer as defined in claim 3 characterized inthat, the cross-sectional shape of said coil is substantially triangularand said one side coincides with an apex of said triangle.
 9. Apuppeteer's glove, characterized in that, each finger and the thumb ofsaid glove has a transducer as defined in claim 1 extending over one ormore knuckle joints.
 10. A puppeteer's glove as defined in claim 9,characterized in that, a separate said transducer extends over each saidknuckle joint.
 11. A puppeteer's glove as defined in claim 9,characterized in that, a single said transducer extends over the threejoints of each finger and a further said transducer extends over thethumb joints.
 12. A puppeteer's glove as defined in claim 9,characterized in that said transducers are on the back of each fingerand the thumb of said glove.
 13. A puppeteer's glove as defined in claim12, characterized in that a read-out circuit including amicro-controller is provided on said glove to receive electrical outputfrom said coil and convert changes in capacitance between the respectiveconductors into signals readable by an external computer.
 14. Apuppeteer's glove as defined in claim 10, characterized in that saidtransducers are on the back of each finger and the thumb of said glove.15. A puppeteer's glove as defined in claim 11, characterized in thatsaid transducers are on the back of each finger and the thumb of saidglove.
 16. A puppeteer's glove as defined in claim 14, characterized inthat a read-out circuit including a micro-controller is provided on saidglove to receive electrical output from said coil and convert changes incapacitance between the respective conductors into signals readable byan external computer.
 17. A puppeteer's glove as defined in claim 15,characterized in that a read-out circuit including a micro-controller isprovided on said glove to receive electrical output from said coil andconvert changes in capacitance between the respective conductors intosignals readable by an external computer.